Gray is close enough to Gary, and makes the card feel more like a friend you're excited to see than one that's come to kill you, something that Gray Merchant of Asphodel often did when it hit the battlefield in mono black decks
However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once
You should expect the same outcome for Ironscale Hydra
So, it incentives you to cast your most considerable colorless spells and rewards you with not one, but two more cards for it
WotC is looking to give you a chance for something a bit unexpected in the world of online spoilers and set lists
Luckily, if it turns out sideways, you are only making this mistake once